3. FISH SPECIES DIVERSITY IN RIVERS, LAKES, AND RESERVOIRS OF NIGERIA (Contd.)

Among the natural lakes and wetlands with documented lists of fish species are:

Oguta Lake: In Imo State supplied with water by the River Njoba. The lake flows into the River Orashi, a
tributary of the lower Niger. A total of about 40 species has been recorded in the lake (Table 4).

Lake Ndakolowo: Located downstream of Jebba Reservoir this is a floodplain lake of the Niger River. The
lake had a surface area of about 9 km2 which was reduced to about 3 km2 after the damming of the Niger at
Kainji and Jebba. The surface area continued to shrink until the whole lake area was covered with marsh
plants and with no visible open water. The National Institute for Freshwater Fisheries Research conducted
a survey in 1978 to identify the inlet and outlet of the lake with a view to recharging the lake by excavating a
channel at its natural inlet. The recommendations of this survey (Ita and Mohammed 1979) were implemented
in 1988 by the Institute and the lake was refilled to give open water extending over 3 km2. The species diversity
recovered from about 9 to 25 species after refilling.

Lake Chad: This is one of the most intensively studied natural lakes in Nigeria. A total of about 80 species of
fish were recorded in the lake and the inflowing rivers by Hopson as reported in Reed et. al. (1967). Since
then, the lake has been seriously affected by the Sahelian drought of the early seventies and eighties leading
to a drastic decline in fish species diversity. The Yobe River, which used to be the main source of water for
flooding the lake along the Nigerian sector, is now broken up into extensive wetland marshes.

A survey conducted in 1985 revealed a total of 19 species (Bukar and Gubio, 1985). This was regarded as an
improvement over the previous years when the lake was virtually reduced to a mass of highly vegetated wetland
swamps with the number of species recorded reduced to about ten.

Hadejia/Nguru Wetlands: This, the most extensive wetland area in the northern part of the country, has
recently gained international recognition on account of the regular presence of migrant birds. It is estimated
to have once covered about 2,000 to 3,000 km2. Between 1964 and 1971 over 2,000 km2 of flooding occurred.
However, from 1983 less than 900 km2 has been flooded and even less than 300 km2 were flooded in the
drought year of 1984 (Adams and Hollis, 1987). In addition to drought it is estimated that the damming of
the Kano River at Tiga has caused a reduction of about 350 km2 in the surface area of the wetland. This
decreased flooding has resulted in a consequent decrease in fish species diversity within the wetland.
Although no pre-drought/pre-Tiga investigations were carried out, the diversity figure (46 species) supplied
in Tobor (1973) for the lower Yobe River is regarded as the most likely situation prior to the drought and
completion of the Tiga dam.

Matthes (1990) records over 40 species (Table 4) for these wetlands, excluding about 30 unidentified species.
This shows that, in spite of the intensive exploitation of the wetland fisheries, a comparatively high diversity has
been sustained. The majority of the species identified are of little or no economic importance on account of their
small sizes, e.g. some small mormyrids and cyprinids which are not usually trapped by fishermen's gear.

A drastic decline has been observed among the larger species such as Lates, Gymnarchus, Heterotis, and
Heterobranchus although not to the point of extinction. The wetlands appear to be the last hope for the
conservation of river fishes. The Hadejia Nguru wetland is unique in this regard and therefore requires special
attention in view of the deplorable impact of drought on Lake Chad.

Goronyo and Bakolori reservoirs on the Rima and Sokoto rivers, respectively, both tributaries of the main
Sokoto River which empties into the upper Niger beyond the confluence with the Kaduna River

Tiga Reservoir is located on Kano River. It flows into the Hadejia River valley which joins up with the Jama'are
River to form the Yobe River that empties into Lake Chad.

Kiri and Dadin Kowa reservoirs are located on the Gongola River, a tributary of the River Benue. No authentic
checklist has been found for Dadin Kowa reservoir, hence its exclusion from Table 5.

Kainji Lake tops the list with a total of 101 species followed by Jebba with 52 species. The high diversity index
recorded for Kainji Lake is not unconnected with the intensity of investigations conducted in the lake since 1969.
Jebba Lake, which extends from the outflow of Kainji Lake for about 100 km to the dam, is expected to harbour
as many species as Kainji Lake if not more. However, on account of the paucity of investigations conducted on
this reservoir, only about half the number of species in Kainji Lake have been documented for Jebba Lake.

Although Kainji Lake still retains some riverine features along its northern arm, a reduction in species diversity
was recorded after impoundment from over 120 species to about 97 species. This was to be expected in view of
the reduction in the flow rate that favoured the behaviour of most cyprinids and cyprinodonts. Some mormyrids
disappeared soon after impoundment, but were later identified along some of the inflowing rivers.

The low species diversity in the other smaller reservoirs can be attributed partly to the fact that the dams are
located nearer to the tributary river sources than to the confluences of the main rivers. Such areas are
characterized by the rapids and rocky terrain preferred by a limited number of freshwater species. The checklist
of the species recorded in these reservoirs is presented in Table 5.